Security is an important concern in the realm of documents and digital production and/or reproduction of the same. Known digital image printing/copying systems produce documents of such high quality that a need has been identified to prevent effective printing/copying of certain documents such as high-value printed items including tickets, financial instruments, security passes, pharmaceutical prescriptions, and the like. Known techniques include printing the original document in a manner such that it includes a “watermark,” using only conventional paper and toner or ink. One such watermarking technique that has been developed and is known to increase document security involves the use and implementation of correlation marks.
Correlation marks offer a unique and simple method for protecting and tracking documents. Based on digital screening technology, correlation marks take advantage of spatial correlations in halftone screens to create a self-authenticating, secure document containing an invisible, hidden image. Using correlation marks, invisible images can be hidden within a normal, visible image, such as a photograph, using specially designed halftone screens. The halftone screens utilized to embed the correlation marks can be used like any traditional halftone screen, no additional image processing is required.
Different from digital watermarking technology, correlation mark technology hides an invisible image during the printing process. No alteration to the original image data is required. And because the hidden mark is inserted during the printing process, each copy of a document can have a unique hidden image.
Hidden images made using correlation marks technology are not visible to the human eye. Such hidden images can be detected either by scanning and processing the printed image or by overlaying a transparency containing a ‘key’ on top of the document to reveal the hidden image. Correlation marks are thus robust to copying and distortion and can be detected in reproductions of the halftone image.
In the area of document security, documents are protected from copying, forging, and counterfeiting using multiple techniques including, but not limited to, watermarking, correlation marks, etc. One approach for document protection involves using standard material such as papers, inks, toners, etc. Typically, however, most security printing companies in the marketplace require special (and expensive) materials. One example of a document requiring security is a prescription where a pharmacist would like to be able to have a high level of confidence that the document is genuine.
The correlation marks shown in FIGS. 1 and 2 are examples of a document security technique that requires a screen or a key to decode. FIG. 1 illustrates a prior art image 1 of correlation marks located above barcodes, and FIG. 2 illustrates a prior art image 2 of correlation marks with respect to a screen.
The approach shown in FIGS. 1-2 generally involves modifying an image to compensate for a lightning effect in later processing steps (the image only needs to be sent to a device one time), creating a pattern ink with the image, adding a vertical and horizontal white space to the image, setting the pattern ink to be the current color (anything written will write in the image with the white space), and writing a gloss (or correlation) mark variable text until the entire image is written. The last step can be repeated N times once per variable data.
One of the primary problems with this approach, however, is that although this technique does work in some production devices and work flows, when implemented in newer applications, this technique fails with an out-of-memory error with respect to target office devices. This problem is caused because the pattern caches (e.g., memory) were not designed for the images from the steps above. Increasing the pattern cache or avoid using it does not solve the issues.
It is therefore believed that an improved correlation mark technique is needed to overcome these problems.